The present invention relates to disc brakes for vehicles, and in particular to a system and method for mounting and retaining brake pads in disc brakes, such as air-operated disc brakes utilized on commercial vehicles.
Pneumatically-operated disc brakes have been undergoing development and deployment on commercial vehicles since at least the 1970's, and are beginning to replace drum-style brakes due to advantages in areas such as cooling, fade resistance and serviceability. German Patent Publication No. DE 40 32 886 A1, and in particular FIG. 1 of the German reference, discloses an example of such an air disc brake. In this design, a pneumatic diaphragm chamber (pneumatic actuator) is attached to a rear face of the disc brake caliper housing, and applies a brake actuation force through a linear actuator rod to a broke actuator lever within the caliper. The brake's actuator lever in turn transfers and multiplies the force applied by the actuator rod to one or more spindles, which force brake pads against a brake disc or rotor. The terms “brake disc,” “rotor” and “brake rotor” are used interchangeably herein.
As shown in FIG. 1 of DE 40 32 886 A1, the actuator is located inboard of the brake caliper, in large part because commercial vehicle wheel rims are sized to only provide adequate clearance for the drum-type brakes historically employed on such vehicles. Because the resulting space envelope between the wheel and its axle is limited, the actuator must be located into the space adjacent to the wheel. For the same reason, brake pads must be configured to conform to the limited available radial space, and thus have typically been located and retained on one of the brake caliper or brake caliper carrier/mount using transverse suspension pins and/or using leaf spring-type metal strips disposed over the outer radius of the brake pads. Brake pads have also been retained by capturing the brake pads between the caliper mounting frame and the portion of the brake caliper which straddles the brake disk. (As one of ordinary skill will recognize, the same brake pad support function may be provided by a brake caliper carrier/mount designed to support the brake pads or by a brake pad carrier which is separate from the caliper mounting structure. For convenience in this description, the terms caliper carrier, caliper mount and brake pad carrier may be interchanged without intending to limit the brake pad supporting structure to any specific brake pad and brake caliper carrying structure.)
Conventional commercial vehicle air disc brakes have typically required the installation of ancillary brake pad retention mechanisms such as a retaining bar arranged transversely across the top of the brake pads. Such arrangements require additional space above the brake pads, which in turn requires taller brake calipers. With the opposing constraint of little available clearance between the inside of the wheel rim and the outer envelope of the brake caliper, it is difficult to accommodate such brake pad retention mechanisms without thinning the caliper and thereby increasing the stress levels in the caliper material and decreasing the caliper's useful service life (e.g., decreased fatigue life).
A further problem with prior art brake pads is the tendency for the brake pad to rotate and/or vibrate during brake operation. As illustrated in FIG. 1, when a brake pad 101 is applied against a friction surface of a brake disk (not illustrated) which is rotating in direction DR, the brake disk's rotation induces motion and reaction forces between the brake pad 101 and its adjacent mount abutment surfaces (not illustrated for clarity). Specifically, at the leading edge 102 of the brake pad the brake pad attempts to move upward in direction LU in response to the friction forces along the face of the brake pad (illustrated here by force arrows across the face of brake pad 101). At the trailing edge 103 of the brake pad, the brake pad attempts to move downward in direction TD. However, because the brake pad 101 is constrained by adjacent mount abutment surfaces, the overall motion of the brake pad is generally a rotation about an axis parallel to the brake disk rotation axis. This motion may be unilateral during the brake application, or may manifest itself as a moderate-to-severe oscillation of the brake pad in its mount, significantly increasing wear of the abutting brake pad and mount surfaces.
In order to prevent undesired rotation and/or vibration of the brake pad within its mounting (for example, rotation about the brake application direction), the brake pad backing plate and the adjacent mounting bracket horns supporting the brake pads in the circumferential direction required a relatively tall radial height to minimize the amount of brake pad rotation before a corner of the backing plate contacted the adjacent mount horn (a motion referred to as “pad kick” or “pad turnout,” caused by “twisting” or “tipping” of the pad in the caliper as the rotating brake disk attempts to raise one end of the brake pad while pushing down on the other end of the brake pad). This relatively tall structure in turn would require the brake caliper, which is installed over the brake pads and mounting bracket, to have its corresponding opposing inner surfaces radially outboard of the mount horns be relieved enough to accommodate the outer corners of the brake pad and/or mount horns. This thinning again may cause stress level and fatigue life problems. Because the maximum outer radius of the brake caliper is typically constrained by very tight clearance to the inside of the adjacent wheel rim, the brake caliper arms straddling the brake disc between the application side and the reaction side of the caliper may have to be made thinner than desired in order to accommodate both the tall carrier mount horn and the close-fitting wheel rim. This can lead to very high tensile and bending stresses in the thin regions and thus reduction in fatigue life and service life.
The present invention addresses these and other problems by providing a brake pad mounting and retention arrangement and method of installation and removal which provides for minimum pad kick motion, elimination of the need for over-topping brake pad retention devices, inherently vibration- and noise-reducing mounting, and lower mount horn height which in turn permits increased brake caliper strength and service life.
In one embodiment of the present invention a brake pad is provided with slots on its lateral and bottom edges arranged to receive a correspondingly-shaped spring element. The spring element in turn has side extensions, such as tabs, which conform to the width of the brake pad carrier and serve to both align and guide the brake pad into position during installation and lock the spring element into place against lateral motion toward or away from the brake disc as the brake pad is advanced or withdrawn from the brake disc. The spring element is further provided with a retention feature on a top surface which may engage a corresponding slot or relief in the pad-side face of the adjacent mount's horn. The retention feature may be an angled tab which permits the brake pad and spring element to be inserted as an assembly vertically downward (i.e., radially inward toward the vehicle axle) in to the mount until the upper retaining feature reaches its corresponding mount relief and elastically springs outward. With this arrangement, during in-service brake operations the brake pad is retained in the brake in a simple and positive manner by the combination of: (i) the upper retention feature snapping into the mount relief; and (ii) the spring element having a protrusion which extends into one of the slots in the edge of the brake pad. A further extension of the spring element which engages the slot on the lower edge of the brake pad may also be provided to assist in retaining the spring element on the brake pad before the brake pad is inserted into the mount.
This arrangement further has the advantage of taking up brake pad-to-mount clearance at the sides of the brake pad, such that brake pad rotation within the mount (“pad kick”) is significantly limited. The limited rotation motion reduces the need for tall mount horns to control brake pad rotation, thereby permitting shorter horns and in turn thicker brake caliper sections above the horns to increase caliper fatigue and service life. The arrangement also eliminates the need for space-taking over-topping brake pad retention devices.
Another advantage of the use of the inventive spring retention element is that it provides a replaceable wear surface which protects the facing surfaces of the mount horns from wear, extending mount service life.
The inventive brake pad and spring retention device may be provided on only one side of the brake pad, or on both lateral sides to further reduce pad rotation motion.
The spring retention device is preferably formed from high-strength material, preferably heat-treated spring steel, and includes a pad vibration-reducing tension tab. Such an element is easily and cost-effectively produced, for example by a metal-stamping process.
The spring element protrusion which engages the side of the brake pad is preferably formed in a wedge-shape which resists “camming out” of the brake pad slot during the brake application.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.